1. Field of the Invention
The present invention relates to a method of reducing particle build-up on a surface recessed from a conduit carrying particles of a solid entrained in a gas.
2. Related Art
Solid materials in particulate form are conveyed pneumatically in a range of industries. Cement raw materials and finished product can be conveyed pneumatically. Similarly, coal is commonly conveyed pneumatically to burners in power station boilers. In the coal fired boilers of the type used to generate electricity, coal is pulverised in grinding mills and transported by heated primary air pipes to individual burners contained within a plurality of similar burners that comprise the burner of a boiler. Maldistribution of fuel across the burner fuel supply pipes has detrimental effects on boiler performance. Fuel rich areas lead to high levels of unburnt carbon in fly ash and fuel lean areas can lead to high levels of NOz. In order to improve the efficiency of boiler performance, it is necessary to monitor the mass flow rate of coal within the burner supply pipes.
International Patent Application Publication No. WO 91/09284 and U.S. Pat. No. 4,882,934 describe meters that are capable of measuring the mass flow rate of particulate material conveyed pneumatically in a conduit. Both devices utilise changes induced in ultrasonic beams transmitted across a conduit carrying particulate material to determine the velocity of flow and concentration of solids and hence mass flow rate of solids in the conduit.
The ultrasonic transducers shown in FIG. 6 and the figure accompanying the abstract of the specification of U.S. Pat. No. 4,882,934 are mounted flush with the wall of the conduit in which they are located, and in operation partially intrude into the conduit. It was envisaged that the flow of particulate material in the conduit itself would maintain the face of the transducers free of particulate build-up.
Tests on arrangements similar to the device described in U.S. Pat. No. 4,882,934 indicated that ultrasonic transducers exposed to the flow of particulate material in a conduit will suffer from several problems. These are as follows:
(i) a variable level of solids adheres to the transducers thereby varying the sound leaving the transmitting transducer and the sound entering the receiving transducer;
(ii) the impact of solids travelling at typically 25 meters per second wears the transducer and degrades its performance; and
(iii) the elevated temperature in the conduit will reduce the signal transmitted by up to 50% and variation in temperature will cause changes in sound entering or leaving the transducer. In addition, the lifetime of the transducers will be reduced at elevated temperatures.
Furthermore, transducers of the type disclosed in the specification of U.S. Pat. No. 4,882,934 can only be replaced if the conduit is taken out of service. Since coal conveying conduits are usually higher in pressure than atmospheric, removal of a transducer from an in-service conduit carrying coal would lead to substantial leakage of particulate coal from the conduit.
The specification of International Patent Application Publication No. WO 91/09284 discloses in FIG. 1, transducers located within recesses. Locating the transducers within recesses reduces transducer wear, but does not prevent the build-up of particulate material. Attempts to prevent the build-up of particulate material on the transducer surface with jets of air succeeded in preventing build-up, but degraded the ultrasonic signal produced by the transducer as a result of turbulence created in the recess.
The present inventors have now discovered that particulate build-up on the face of an ultrasonic transducer located in a recess can be prevented by introducing gas into the recess at relatively high volume, but low velocity. Furthermore, the volume of gas required is very low with respect to the volume of gas flowing through the conduit.
Accordingly, the present invention provides a method of reducing particulate build-up on a surface recessed from a conduit carrying particles of a solid entrained in a gas, the method including blowing gas into a recess containing the surface at high volume and low velocity so that gas entering the recess flows in a substantially laminar manner.
The present invention also provides a housing for attachment to a conduit, the housing defining a recess when attached to the conduit in which a device such as an ultrasonic transducer can be mounted and including first inlet means for introducing gas into the recess in such a way that the gas flows at high volume and low velocity in a laminar manner across a face of the device located in the recess.
The housing may also include second inlet means for introducing gas at high pressure and high velocity into the recess.
Gas introduced into the recess via the first inlet permits the face of the device located in the recess to remain free of particulate build-up. It also can be used to cool the device. The high pressure inlet permits devices to be changed whilst the conduit is in service. Thus, in a preferred form, the invention allows compressed gas to be delivered in two different ways:
(i) as a high velocity stream directed into the recess; and
(ii) a high volume, low velocity stream directed onto the face of the device.
The high velocity jet directed into the recess enables interchange of a device while the conduit is in service. The second gas stream enables cooling of the device and maintains the face of the device free of particulate matter.
The way in which air is delivered to the face of the device may be critical to its performance. Taking an ultrasonic transducer as an example, delivering a high velocity jet of air onto the face of the transducer cleans the face but reduces the efficiency of the ultrasonic beam transmitted or received by the transducer as a result of turbulence. The present invention permits the face of the ultrasonic transducer to remain free of particulate matter whilst minimising the disturbance of the ultrasonic transmission.
In order to ensure laminar flow of gas across the face of the transducer and through the recess, gas is preferably introduced into the recess via a calming chamber that reduces the velocity of the incoming gas to that at which laminar flow prevails.